1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems related to seismic exploration and, more particularly, to mechanisms and techniques for providing a broadband seismic source.
2. Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of a geophysical structure under the seafloor. While this profile does not provide an accurate location of oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of these reservoirs. Thus, providing a high-resolution image of the structures under the seafloor is an ongoing process.
During a seismic gathering process, as shown in FIG. 1, a vessel 10 tows an array of seismic receivers 11 provided on streamers 12. The streamers may be disposed horizontally, i.e., lying at a constant depth relative to a surface 14 of the ocean. The streamers may be disposed to have other spatial arrangements than horizontally. The vessel 10 also tows a seismic source array 16 that is configured to generate a seismic wave 18. The seismic wave 18 propagates downward, toward the seafloor 20, and penetrates the seafloor until eventually, a reflecting structure 22 (reflector), reflects the seismic wave. The reflected seismic wave 24 propagates upwardly until it is detected by the receiver 11 on streamer 12. Based on this data, an image of the subsurface is generated.
In an effort to improve the resolution of the subsurface's image, an innovative solution (BroadSeis) has been implemented based on broadband seismic data. BroadSeis may use Sentinel streamers (produced by Sercel) with low noise characteristics and the ability to deploy the streamers in configurations allowing the recording of an extra octave or more of low-frequencies. The streamers are designed to record seismic data while being towed at greater depths and are quieter than other streamers. Thus, the receivers of these streamers need a marine broadband source array.
Marine broadband source arrays may include plural source points provided along an X direction as shown in FIG. 2. Such a source array includes a float 30 that may be connected to a vessel (not shown) via a connection 32. The float 30 is configured to float at the surface of the water or near the surface of the water and to support plural source points 34. Source points 34 are suspended with appropriate cables 36 from the float 30 and also might be connected to each other by cables 38. An umbilical cable 40 may link one source point 34 to the vessel for providing a mechanical connection, and also electrical, pneumatic and/or communication cables. Source points 34 are typically provided at a same depth from a surface of the water.
One disadvantage of such a source array is that, depending on the type of survey, the size of the source array 16 is too large. Although FIG. 2 shows only three independent source points 34, a typical source array may have around 30 source points with the source points provided in sub-arrays, e.g., seven source points along a straight line. Reducing the length of the source array is not an easy task because by reducing the number of sources (e.g., airguns), the diversity of the source array is impacted, which sequentially reduces the quality of the source array and its tuning. Thus, simply reducing the number of source points to reduce the overall size of the source array is not a solution.
An alternate source array is discussed in WO 2009/005939, the entire content of which is incorporated herein by reference. This reference discloses using plural floats 40 floating at the surface 42 of the water as shown in FIG. 3. There are sub-arrays that include individual sources 44 provided at a first depth z1 and sub-arrays that include individual sources 46 provided at a second depth z2, larger than z1. However, such a configuration is still sizeable and necessitates a large number of floats.
Thus, the existing source arrays, due to their large size, have a large azimuthal footprint, i.e., not a good directionality. In order to obtain more precise images of the subsurface, it is desired that the source array is more omnidirectional, i.e., has a reduced azimuthal footprint. In other words, the illumination produced by the source needs to be more focused. This will extend the high-frequency energy spectrum, and will make this spectrum smoother. At the same time, it is desired to provide an economical and reliable airgun mechanical arrangement that is compatible with existing 3-dimensional seismic vessels.
Another problem that affects the conventional sources is “ghost reflections.” Ghost reflections occur when upwardly travelling seismic energy is reflected or scattered downwards at the sea surface. The ghost reflections are also detected by the seismic receivers and generate notches in the recorded data. Various solutions have been proposed to address this matter but, at this time, no approach is highly effective. Accordingly, it would be desirable to provide systems and methods that provide a source array having a reduced footprint and improve the broadband characteristics of the recorded data.